I. FIELD OF THE INVENTION
This invention relates to the field of automatic gain control circuits.
II. BACKGROUND ART
Automatic gain control (AGC) circuits are used to maintain a constant amplitude signal output level regardless of the level of an input signal. In other words, although the amplitude of an input signal level may vary, the output of the AGC circuit is a constant amplitude. Whereas a typical electronic amplifier accepts an incoming electronic signal and amplifies that signal by a fixed amount, an AGC provides only enough amplification to reach the desired output amplitude level.
One use for AGC circuits is in communication receiving channels, such as found in a modem or similar application. Input signals on a telephone line, varying in amplitude, are inputted to the AGC circuit. It is desired that the input signal be amplified to a fixed level for subsequent processing and decoding. Therefore, the AGC amplifies the input signal to a fixed output level.
In the prior art, a single AGC stage is used for controlling the level of an input signal. In operation, the gain of an AGC circuit amplifier stage varies inversely to the amplitude of the input signal level. As the amplitude of the input signal increases, the amount of gain is reduced. The AGC is initialized at a predetermined reference level which the AGC seeks to maintain. When amplitude of the input signal falls below this predetermined threshold level, the AGC circuit senses the decrease in amplitude at the output of the amplifier. The AGC circuit then increases the amplifier stage gain until the amplitude of the output signal increases to the reference level. If the amplitude of the input signal is above the nominal reference level, the AGC circuit senses the increase in amplitude at the output of the amplifier stage and decreases the amplifier stage gain until the amplitude of the output signal decreases to the reference level.
The amount that the gain stage in a digitally controlled AGC can be increased or decreased is "stepped". In other words, each change in gain is taken in discrete steps. Thus, if an input signal level is below the desired reference level, the amplifier gain is increased one level at a time until the input signal level is "captured", that is matched by the increased gain.
A signal input line is subject to "transients", large spikes of noise which may be of very short duration with a stepped AGC design, if the transient is much larger than the individual increments in gain stage level, then there could be an unacceptable amount of time spent in "capture" of the incoming signal. In addition, if the amplitude falls rapidly, there could be a long "settling time" while the gain stage is stepped down to the proper level. During this settling time, the output of the AGC circuit is not accurate, resulting in incorrect data being outputted to the remainder of the circuit. Therefore, large transients or noise may cause long bursts of data error. If the gain increments are made large enough to handle large transients in a minimum amount of steps, then small transients will typically be ignored in order to avoid overshoot.
In other prior art schemes, AGC circuitry attacks the input signal by tracking signal envelope. This method of tracking is sensitive to data power modulation. As a result, the method of modulation influences the type of tracking algorithm utilized by the AGC circuit.
Therefore, it is an object of the present invention to provide an automatic gain control circuit which is independent of the data modulation technique of the input signal.
It is a further object of the present invention to provide an AGC having a reduced settling time.
It is still another object of the present invention to provide an AGC which has reduced transient amplitude sensitivity.
It is yet another object of the present invention to provide an AGC for tracking small amplitude disturbances.